This invention relates generally to a high intensity discharge (HID) lamp ballast and, more particularly, to an HID lamp ballast having a transient protected power factor correction scheme.
An HID lamp generally refers to a family of lamps including high pressure mercury, high pressure sodium, metal halide and low pressure sodium. A conventional ballast for powering an HID lamp often includes a capacitive power factor correction scheme which is responsive to and employed when the HID lamp is lit. The power factor correction scheme is not used when the HID lamp is not lit. Otherwise, a relatively high current level drawn by the ballast prior to the lamp being lit will limit the number of power factor correction ballasts which can be connected to a branch utility power line (i.e. protected by a circuit breaker).
The responsiveness of the power factor correction scheme to the HID lamp status (i.e., lit or not lit) can be provided by a switching device, the switching device being turned ON and OFF based on the HID lamp status. When the HID lamp is lit, the switching device (e.g. a thyristor such as a triac) is turned ON which permits the power factor correction scheme to draw capacitive current thereby improving the ballast power factor.
Power line transients (i.e. transients having voltages substantially above the normal peak ballast input voltage), however, are often above the voltage breakover of the triac resulting in the triac being turned ON when it should be turned OFF. Under such conditions, current crowding (i.e. carrier current concentrated in a narrow stream) within the triac can occur. Overheating of the triac silicon material and subsequent failure of the triac can result.
Accordingly, it is desirable to provide an improved HID ballast having a more reliable power factor correction scheme. The power factor correction scheme, in particular, should be protected from power line transients to more reliably control when the scheme is employed.